Process chambers for thermally processing semiconductor wafers are desirably made of quartz (vitreous silica) or similar material because quartz is substantially transparent to radiant energy. Thus, radiant heaters may be positioned adjacent the exterior of the chamber, and a wafer being processed in the chamber can be heated to elevated temperatures without having the chamber walls heated to the same level. On the other hand, quartz is desirable because it can withstand very high temperatures. Quartz is also desirable because of its inert characteristics that enable it to withstand degradation by various processing gases and because of its high purity characteristics.
For applications in which the pressure within a quartz chamber is to be reduced much lower than the surrounding ambient pressure, cylindrical or spherical chambers are preferred from a strength standpoint because their curved surfaces can best withstand the inwardly directed force. However, when positioning a flat wafer for chemical vapor deposition purposes where the deposition gases flow parallel to the wafer, it is desirable that the chamber wall be parallel to the facing flat surface of the wafer, to obtain even deposition on the wafer surface. Uniform deposition is critical to obtain a high yield of acceptable products to be made from such wafer. However, a flat wall will collapse inwardly with reduced interior pressure sooner than will an outwardly convex wall of similar size and thickness.
To handle the inwardly directed forces oil flat wall chambers, gussets have been provided on the exterior of the walls extending generally perpendicular to the walls to which they are joined, as may be seen in U.S. Pat. No. 4,920,918. That patent also illustrates gussets on the exterior of a chamber having upper and lower outwardly convex elliptical walls having a large radius of curvature, thus providing a somewhat flattened, but curved, configuration. This compromise provides some additional strength from the curved walls while not affecting the evenness of deposition appreciably. One significant disadvantage of such design is that the external gussets complicate and interfere with the external positioning of radiant heat lamps. Furthermore, the complexity and mass of the quartz gussets increases material and fabrication expense.
Of course, flat walls can be made thicker to increase strength, but that adds cost and adversely affects heating and cooling characteristics of the chamber.
U.S. Pat. No. 5,085,887 discloses a chamber which includes a circular, slightly domed, or curved upper chamber wall to accommodate the load of reduced chamber pressure. The circular wall is provided with a greatly thickened peripheral flange that radially confines the upper wall to cause the domed wall to bow outward due to thermal expansion, helping to resist the exterior ambient pressure in vacuum applications. The chamber requires a complex mechanism for clamping the thickened exterior flanges of the upper and lower chamber walls.
Due to the high temperatures associated with thermally activated chemical vapor deposition processes, the walls of the process chamber often heat up to a certain degree, and chemical particulates are deposited thereon. These particulates can cause serious problems with the purity of the resulting processed wafer. As a result, there has been a large effort to reduce the buildup of particulate matter on reaction chamber walls. One solution is to periodically etch the insides of the process chambers to remove the particulate matter before it accumulates to a harmful level. Unfortunately, quartz process chambers take a long time to heat up due to their high transparency to radiant heat. These periodic slow etch cycles thus reduce the maximum throughput of the machine.
There has also been attempts at controlling the gas flow profile in parallel across the wafer to be processed so as to create a more uniform deposition. For example, U.S. Pat. No. 5,221,556 discloses a system in which the apertures through a gas inlet manifold are varied in size to allow more gas through one section, typically the center section, as opposed to others. U.S. Pat. No. 5,269,847 includes valves for adjustment of pairs of gas flows merging into a number of independent streams distributed laterally upstream of the wafer to be processed. This system emphasizes the importance of channeling the various gas flows separately until just before the wafer leading edge so as to prevent premature mixing and enable greater control over the flow and concentration profiles of reactant and carrier gases across the wafer.
Another problem which has not been sufficiently addressed in the prior art is that of recirculation of the process gas in parallel flow reactors. More particularly, after the gas travels in parallel over the wafer and susceptor, it may experience temperature gradients between the hot susceptor and cooler chamber walls. This can lead to recirculations as the gas rises toward the walls and is subsequently cooled. Also, the gas flow may be constricted proximate an exhaust conduit which may create turbulence and recirculations. Recirculations from either source may migrate upstream to impact the uniformity of flow in the area of the wafer thus reducing the uniformity of film deposition.
Additionally, the temperature gradient across the wafer is nonuniform from the leading edge to the trailing edge. That is, the temperature of the gas is primarily determined by its proximity to the heat-absorbing susceptor underneath the wafer. As the gas approaches and passes over the susceptor, it heats up fairly quickly to a maximum temperature towards the downstream edge of the susceptor, and then drops off after traveling past that point. This temperature nonuniformity may further negatively affect film deposition uniformity.
A need exists for an improved chamber for chemical vapor deposition purposes, and other high temperature processes, that can be made of quartz or similar materials and yet withstand the stresses incident to reduced pressure processes. There is also a need for a more uniform temperature and flow environment surrounding the wafer to ensure more uniform deposition thereof. Also, a more responsive flow control system is needed. Finally, there is a need for a more energy efficient chemical vapor deposition system with higher throughput.